Numerous applications of micro-electromechanical (MEMS) inertial sensors require a high-performance ASIC interface. Existing interface techniques are not fully satisfactory in various respects.
For example, in feedback (e.g., force-feedback) systems where a drive loop is present, an excitation signal is needed for detection of capacitance variations in both a sense loop and a drive loop. The excitation signal should not affect the actuation applied, for example, to a proof mass (or proof masses) of the MEMS sensor. However, since the excitation signal is applied to the proof mass, and since actuation capacitors share the same proof mass with the detection capacitors, therefore the excitation signal affects the actuation signal content and dynamic range.
Another issue relates to undesired coupling that can occur between the actuation stream of one channel and detection paths of the same channel, or even different channels (e.g., sense mode to sense mode coupling or sense mode to drive mode coupling, etc.). Such coupling can distort the signal and result in severe degradation in the performance of the detection front-end circuits. This effect is exaggerated in sense mode, as the combined effect of parasitic capacitance and process mismatch is on the order of the detection capacitance variation.
Several solutions have been proposed to solve this coupling issue. Some solutions depend on frequency separation between actuation and detection (which works only in the case of coupling between different channels); other solutions depend on estimating the coupling transfer function and compensating this effect in later stages (in digital domain signal processing or—at the cost of increased complexity—in the analog domain). Other proposals have included decreasing the actuation signal level (at the expense of reduced actuation dynamic range), or manual trimming to compensate for the mismatches.
In feedback (e.g., force-feedback) systems where a drive loop is present, a sense signal may contain a desired sensor input signal AM-modulated at the frequency of a drive signal. Hence, to demodulate the bit stream to get the original signal, the drive and the sense signals are multiplied using a multiplier to obtain a demodulated output signal. To get the demodulated signal to have the lowest possible DC component, accurate phase adjustment between SNS and DRV bit streams may be required. Various approaches to achieving this phase adjustment typically entail power and/or area penalties.
Hence, an improved interface for interfacing to MEMS inertial sensors is desired.